Pulse frequency modulated power supplies are common for producing the fixed voltage DC output required to charge battery powered mobile devices from a variety of AC inputs. While smaller, more flexible, and more efficient than simple transformer power supplies, pulse frequency modulated power supplies are still active mechanisms that consume power. With the growth of mobile devices, now numbering in the billions, cumulative standby power consumption by charging power supplies is becoming a problem. The problem being that power supplies that remain attached to a power source consume standby power in order to remain active and achieve regulated output voltage.
Furthermore, power supplies that remain attached to both a power source and to a charged device consume additional standby power because the coupled device continues to draw trickle power, even when fully charged. This wasted power is costly to both individual device owners and to society as a whole.
FIG. 1 illustrates a prior art fixed voltage power supply using DC/DC pulse frequency modulation. The high voltage DC current, which can be created by rectifying high voltage AC current (not shown), powers charge pump oscillator (101) operating at a high frequency (typically 50-150 kHz).
The charge pump oscillator (101) controls the gate of MOSFET (102), which in turn, induces a high frequency, high voltage AC signal from the DC supply current through transformer (103). Transformer (103) outputs a magnetically isolated high frequency, low voltage AC signal, rectified by diode (104). Rectifier (104) feeds bulk capacitor (105), which removes ripple from the low voltage rectified signal, producing a DC output.
The Zener diode network (106) allows current to flow through the LED of optical isolator (107) when the output voltage exceeds the Zener threshold. Optical isolator (107), when illuminated, disables the output of the AC waveform from charge pump oscillator (101) to MOSFET (102) and through transformer (103). With charge pump oscillator (101) disabled, output voltage will begin to fall until current no longer flows through Zener diode network (106) and optical isolator (107), allowing charge pump oscillator (101) to become active again.
The resulting pulse frequency modulated waveform (108) generated by charge pump oscillator (101) and feedback network (104-107) is enabled whenever the regulated output voltage is below the regulated threshold and disabled whenever the regulated output voltage is above the regulated threshold. The result is a fixed DC voltage output from an arbitrary high voltage AC input.